The current variable capacity pumps (or motors, which are to be referred to similarly hereinafter) only have two types, namely the reciprocal plunger type and single action rotary vane type of variable capacity pumps. The plunger type variable capacity pumps are extremely sensitive to dust in oil and cost very much. The performance parameters of single action vane type variable capacity pumps are very poor. Furthermore, the pump or motor effect resulting from current hydraulic pumps or motors themselves is not changeable. In order to realize some specific functions in practical applications, some additional hydraulic elements are therefore needed for regulation purposes, thus leading to a huge system with complicated components and high costs. Consequently, the application field of hydrostatic transmitting systems is limited. For instance, hydro-mechanical transmitting systems are still used in current vehicles, although they have three disadvantages: first, the transmitting efficiency is low; secondly, the structure is complicated and the cost is high; thirdly, power cannot be transmitted inversely. The fundamental cause leading to these disadvantages lies in the vortex type hydraulic torque converter (composed of an impeller, a guide torus and a turbine) used in the system. This torque converter operates with high efficiency only in a very narrow range of speed ratio, beyond which the transmitting efficiency will decrease rapidly. The torque converter cannot transmit power inversely.